1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device and a driving method thereof. Specifically, the present invention relates to a solid-state imaging device including a plurality of pixels provided with photosensors and to a method for driving the solid-state imaging device. Further, the present invention relates to an electronic device including the solid-state imaging device.
Note that one embodiment of the present invention is not limited to the above technical field. For example, one embodiment of the present invention relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Further, one embodiment of the present invention relates to a memory device, a processor, a driving method thereof, or a manufacturing method thereof.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. Thus, a semiconductor element such as a transistor or a diode and a semiconductor circuit are semiconductor devices. A display device, a light-emitting device, a lighting device, an electro-optical device, a solid-state imaging device, an electronic device, and the like may include a semiconductor element or a semiconductor circuit. Therefore, a display device, a light-emitting device, a lighting device, an electro-optical device, a solid-state imaging device, an electronic device, and the like include a semiconductor device in some cases.
2. Description of the Related Art
A photosensor using an amplification function of a metal oxide semiconductor (MOS) transistor, called a complementary metal oxide semiconductor (CMOS) sensor, can be fabricated through a general CMOS process. Thus, manufacturing cost of an imaging device including a CMOS sensor in each pixel can be low, and a semiconductor device having a photosensor and a display element formed over one substrate can be realized. Further, the drive voltage of a CMOS sensor is lower than that of a charge coupled device (CCD) sensor, so that power consumption of the solid-state imaging device can be kept low.
A solid-state imaging device including a CMOS sensor generally employs, for imaging, a rolling shutter method in which an operation to accumulate charge in a photodiode and an operation to read the charge are sequentially performed row by row (see Patent Document 1). In some cases, such a solid-state imaging device employs a global shutter method in which all the pixels are subjected to an operation to accumulate charge at a time, instead of the rolling shutter method.
Non-Patent Document 1 discloses an example in which three-dimensional imaging of an object is performed by detecting light reflected from the object.
As Non-Patent Document 1 describes, the channel width of a transistor, which transfers charge obtained through photoelectric conversion of a photodiode to a charge accumulation region, is increased to improve the transfer efficiency.